Hypothermia is routinely induced by physicians to protect the heart and brain of patients during cardiac surgery or operations involving cerebral blood vessels. Physicians may also rapidly cool a patient's body to protect brain tissue following traumatic injury, during resuscitation from cardiac arrest, and to help prevent brain damage after a stroke. In other instances, the rapid warming of a patient can be important, e.g., in cases where hypothermia has resulted from an accident.
At present, cardio-pulmonary bypass (CPB) is the most effective method for rapidly changing a patient's core temperature. However, CPB is invasive and requires sophisticated equipment and well trained personnel. Non-invasive approaches to changing core temperature currently in use rely upon surface cooling or heating by covering a patient's body with a blanket in which either air or water is circulated.
Another approach has been to use the respiratory system for heat exchange. Because liquids generally have higher specific heats than gases, ventilation of patients with a liquid provides one attractive alternative for controlling body temperature (see Forman, et al., J Surg. Res. 40:36–42 (1986)). However, minute ventilation with a liquid is limited by its high viscosity and this, in turn, leads to severe CO2 retention by patients. Moreover, liquids tend to wash out surfactants from the alveoli of lungs, thereby causing injury.
These problems are avoided when a gas is used for inhalation. However, the specific heat of gases tends to be low and, consequently, heat exchange is relatively slow. A significant advance in methods of controlling body temperature would result from the development of procedures in which patients inhale mixtures having both a high specific heat and low viscosity.